UNS: Taking Advantage of Metal Interpenetration to Improve the Performance of Conjugated Polymer/Fullerene-Based Photovoltaics

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PI: Benjamin J. Schwartz Proposal Number: 1510353The sun represents the most abundant potential source of sustainable energy on earth. Solar cells that use light-absorbing organic polymers to convert light to electricity - organic photovoltaic (OPV) devices - offer a potentially low-cost route for renewable electricity production. However, in order to achieve parity with other solar photovoltaic technologies, organic solar cells must increase their power conversion efficiency past the current 10.5% world record. The goal of this project is to add metal nanoparticles into the active layer of the organic solar cell to possibly enhance solar energy conversion efficiency. The active layer of the OPV device consists of the conducting polymer and nanostructured carbon. If the metal nanoparticles are in the right place, they will enhance light absorption, and ultimately solar energy conversion efficiency, through a complex process called plasmonic resonance. A key innovation of this project is that that metal nanoparticles will be formed at precise locations within the active layer to enable this process. As part of the educational activities associated with this project, the principal investigator participates in a program where nanotechnology topics are brought to high school classrooms throughout the greater Los Angeles area in a series of graduate student-run workshops for high school teachers. The graduate student supported by this grant will develop and conduct workshops on solar energy featuring organic solar cells.A major technical challenge with the fabrication of organic photovoltaic devices is to precisely control the nanoscale spatial distribution of the light-absorbing polymer (electron donor) and fullerene (electron acceptor) to optimize charge separation and photocurrent collection. Furthermore, other complicating factors may occur during device fabrication. For example, conductive metals are deposited on the top of the organic polymer layer by thermal evaporation to serve as an electrical contact. It is hypothesized that these evaporated metals easily move through the fullerenes and leave a layer of metal nanopart¬icles underneath any fullerenes that reside at the top of the polymer layer, creating unintended consequences to device performance that have been overlooked to date. Preliminary data supports this hypothesis, and the overall goals of this proposed research are to determine the effects of metal interpenetration on the performance of polymer-based PV devices, and then develop strategies to purposely manipulate metal nanoparticle interpenetration to improve device performance through plasmonic optical absorption enhancement. In the proposed research, experimental and computational approaches will be used to understand these processes. Sequential processing, where the donor (conducting polymer) and acceptor (fullerene) layers are deposited in separate steps, will be used to control the vertical fullerene distribution within the OPV active layer. Transmission electron microscopy, in combination with ellipsometry and neutron reflectometry, will be used to study the conditions by which metals penetrate through fullerenes. This information will be used to develop synthesis strategies to control the distribution of interpenetrated metal, and ultimately the size and position of the metal nanoparticles that are formed. The dielectric constant, plasmonic absorption enhancement, and exciton quenching measurements will performed on the metal nanoparticle impregnated active layer, and these fundamental optoelectronic property measurements will be correlated to overall measurements of device performance, including solar energy conversion efficiency, external quantum efficiency, and photocurrent/photovoltage transients. Complementary simulations that couple full solutions of Maxwell?s equations with standard drift-diffusion solvers will be performed with metal nanoparticles in the active layer to help to understand and interpret these experimental results. The research outcomes will suggest purposeful ways to simultaneously enhance OPV device fabrication and performance.